1. Field of the Invention
The present invention relates to a method of forming a cylindrical boss on a base plate and a die for forming the boss, and more particularly to a method of forming a center boss having a flange for holding a bearing therein and a die for forming such a center boss.
2. Description of Related Art
As a method of manufacturing a cylindrical boss having a small step therein from a steel plate, a method of controlling a thickness and a shape of a metal plate (FCF method) is generally known. The CFC method utilizes various processes such as fixing in cold-forging, ironing, and extruding in addition to conventional processes such as stamping, drawing, bending and fluing.
A method of making a bulge at a foot of a cylindrical boss and a die for use in the method are disclosed in JP-A-7-124657, a part of which is illustrated in FIG. 8 attached hereto for reference purpose. Referring to FIG. 8, the die includes a lower die 35, an upper die 38 and a punch 30. A work having a cylindrical boss 32 is held between the lower die 35 and the upper die 38. The punch 30 is pressed into an inner bore of the cylindrical boss 32 to form a bulge 34 at a base portion 33 of the cylindrical boss 32. The punch 30 is rod-shaped and has a front surface 36 connected to a main rod by a tapered portion 31. A corner having a small radius r is formed at a foot of the cylindrical boss 32 at the same time the bulge 34 is formed by pressing down the punch 30. The bulge 34 is formed along a depressed curve of the punch 30, and the corner with the radius r bulges out into a space 37 between the lower die 35 and the punch 30. At the same time a depression 39 is formed at an outside foot of the cylindrical boss 32. The depression 39 is undesirable because a crack develops at the depression 39 when the bulge 34 is further processed to form a flange. The crack developed at the outer foot of the cylindrical boss 32 is detrimental to a mechanical strength of the cylindrical boss. Because of this, the flange in the cylindrical boss is separately made and assembled into the boss in a conventional process. Accordingly, a production cost for making the cylindrical boss becomes expensive.